# Tag Info

18

There are numerous misconceptions here, but allow me to address just a few: Black holes do not have "appetite." In order for an object to be consumed by a black hole, the object's trajectory would need to literally intersect with the event horizon (i.e. be on a collision course with it), otherwise the object will continue to orbit the black hole. Because ...

14

Let me attempt a more "popular science" answer (Ron please be gentle with me!). In GR a geodesic is the path followed by a freely moving object. There's nothing especially complex about this; if you throw a stone (in a vacuum to avoid air resistance) it follows a geodesic. If the universe is simply connected you'd expect to be able to get anywhere and back ...

10

You first have to understand what a "white hole" is. It's the time reverse of a black hole. It was rightly pointed out in previous answers that white holes violate the second law of thermodynamics. Now, like anything in thermodynamics, this makes them unlikely but not impossible (unlikely here usually means unlikely even in an astronomical number of ...

7

Could the black hole in the center of the galaxy be a white hole? I think not. IMHO there are no white holes. IMHO white holes are a mathematical fantasy. In the center of the galaxy there is a strong radio source which we call Sagittarius A*. Based on the high speed and orbit of nearby stars we have calculated that something with the mass of more ...

6

In 1974 Stephen Hawking published a paper that provides a theoretical basis for the thesis that black holes eventually may radiate away all the mass, light, and other energy they accumulate. Evaporation of black holes has been called Hawking radiation. It takes place so slowly (at least until the black hole shrinks to a small size) that none has been ...

5

No. It's only a fictional object that agrees with einstein's formulas. But there is no evidence that that objects exists or had existed.

5

It's not exactly a duplicate, but have a look at my answer to Entering a black hole, jumping into another universe---with questions. For certain types of black holes it's possible to find a trajectory that takes you inside the event horizon then back out again, but when you emerge you'll find there is no way (without travelling faster than light) to get ...

4

Over the years there have been suggestions that elementary particles may be black holes. However no-one has ever been able to make this quantitative and I doubt anyone believes it these days. There was some discussion of this in what is the difference between a blackhole and a point particle, and Googling will find you lots of hits on this subject. I've ...

4

It's a common claim that certain types of black holes provide a gateway to a parallel universe, however there are two problems with this claim. Firstly, although it's true that trajectories can be traced through the event horizon and back out again, it isn't clear whether this is physically meaningful or just a mathematical trick. In fact if recent ...

4

I don't know the answer for the case with two horizons (non-extremal case), but when there is only one (that is the extremal case: Q=M in suitable units and common notation for charge and mass), here is how it works. Let us first note that in the extremal case, the timelike coordinate outside the black hole remains timelike inside it. This is a major ...

4

Assuming no energy input, the lifetime of a black hole is related to its mass by: $$T = \frac{5120\pi G^2}{\hbar c^4}M^3$$ There is a nice summary of the derivation of the lifetime on this web site. I make the condition assuming no energy input because for large black holes the Hawking temperature is less than the temperature of the cosmic microwave ...

4

I'd like to address the misconceptions in your quote from the wikipedia article. ”Like black holes, white holes have properties like mass, charge, and angular momentum. They attract matter like any other mass, but objects falling towards a white hole would never actually reach the white hole's event horizon" They do "have" mass, charge, and angular ...

3

No. Courtesy of Star Trek, and numerous other Sci-Fi films and books, people tend to have the idea that anti-matter is somehow weird and mysterious. Anti-matter is just matter - it's anti only in the sense that a particle and anti-particle can react to produce two photons. The stress-energy tensor that is the source of spacetime curvature makes no ...

3

Dear Kahtrijn, white holes are microscopically the same objects as black holes, and it's guaranteed - by the second law of thermodynamics - that all macroscopic processes occur in the way as they do in black holes and not white holes (the latter are time-reversed of the former). However, if you don't care about the second law of thermodynamics that prevents ...

3

The point is that the configuration that you are describing as the time-reversed Oppenheimer-Snyder collapse would need very specific initial conditions to occur. A collapse process on the other hand is very generic. You can have a collapse with any number of initial configurations. In that sense a collapse process is more physical than the reverse. It is ...

3

A short answer is that frames themselves are moving towards the black hole and light moves relative to a frame and hence it can be stuck. Nothing passes through a black hole. Things can enter a black hole, they can't can't exit without going faster than light. Where do things go then? The important part of that question is the word "where" you ...

3

In addition to what @Jack Mazy said about force due to gravitational attraction, there are also charged (Reissner–Nordström) black holes. I would imagine in theory you could take another charged object and place it near the black hole which would cause Coulomb force on black hole putting it in motion.

2

So, I brought this up at my research group meeting this week. Turns out my initial guess was right--the past development of the Oppenheimer-Snyder spacetime does contain a white hole. When people say that the matter distribution cuts off the white hole, what they generally assume is that the spacetime does not contain a moment of time symmetry, and that ...

2

This model you are talking about, parallel universes and black holes connecting with them is a popularized version of a model that is not mainstream physics. Black holes exist because we have seen their influence, we have not seen any white hole sources of particles, this is an experimental fact. This is the reason that the black holes of mainstream ...

2

No, physically, no new Universes ever get formed. In particular, the extended Penrose causal diagrams with new infinite regions just show the maximal extension of the spacetime that is possible mathematically, ignoring physical processes inside the black hole. In physics and reality, the extension is unphysical because the naive extrapolation by Einstein's ...

2

White hole is an impossible object in universe. Mathematically it is a black hole under inverted time. This can be interpreted as a black hole in an universe where second law of thermodynamics is inverted, that is the entropy always diminishes. Since second law of thermodynamics has probablistic nature, one can see a white hole as a highly unprobable ...

2

Black holes are not really as exotic as popular media makes them out to be. Modern physics does not predict them to be "gateways" to other universes. Any matter that falls into a black hole gets crushed in the singularity at the center, and will remain there. It's true that anything falling into a black hole becomes causally disconnected from the rest of ...

2

OT, but are you related to Adam Ondra, the rock climber? That guy is an animal. In most areas of physics, we're accustomed to having global symmetries such as parity and time-reversal. These are interpreted as arising from symmetries of the fixed spacetime background, which is Minkowski space. In GR, we aren't guaranteed to have any such symmetries. In fact,...

2

QFT states, that e.g. $e^+ e^-$ pairs are produced and annihilated all the time. Now is is possible, that such a pair is created at the event horizon (EH) of a black hole (BH) and one particle crosses the EH. Since the two cannot annihilate, the particle on our side of the EH lives on and gets away from the BH as Hawking radiation. However, through this ...

2

No one knows what happens inside the black hole (I mean inside the event horizon). But inside the event horizon space becomes unidirectional (like time in real world) and therefore whatever enters into it must hit the singularity inside (at least according to classical general relativity). But again no one knows what happens at that singularity. By ...

2

Nothing special is happening! Think of a black hole as the accumulation of mass which is exceeding a certain limit. The same laws of gravity are applying before and after exceeding the limit. That means: Mass particles keep on being attracted. They are becoming part of the mass of the black hole. Electromagnetic waves will equally be attracted by the mass ...

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